Rheological Properties of Acid Gels Prepared from Heated pH-Adjusted Skim Milk

Abstract

Reconstituted skim milk was adjusted to pH values between 6.5 and 7.1 and heated (90 degrees C) for up to 30 min. The skim milk samples were then readjusted to pH 6.7. Acid gels prepared from heated milk had markedly higher G ' values, a reduced gelation time, and an increased gelation pH than those prepared from unheated milk. An increased pH at heating decreased the gelation time, increased the gelation pH, and increased the final G ' of acid set gels prepared from the heated milk samples. There were only small differences in the level of whey protein denaturation in the samples at different pH values, and these differences could not account for the differences in the G ' of the acid gels. The levels of denatured whey protein associated with the casein micelles decreased and the levels of soluble denatured whey proteins increased as the pH at heating was increased. The results indicated that the soluble denatured whey proteins had a greater effect on the final G ' of the acid gels than the denatured whey proteins associated with the casein micelles.

Effect of heat treatment at alkaline pH on the rennet coagulation properties of skim milk

“…The analysis of the micellar phase of the same milk samples did not, however, indicate that less whey protein/κ-casein aggregates could be bound to the casein micelles as heat-treatment pH increased. Earlier studies using the same method, however, reported a clear correlation between a lower heat-treatment pH and a higher proportion of micelle-bound whey protein/κ-casein aggregates [3][4][5]. The results of the present study were accounted for by the mild heating regime.…”

How to tailor heat-induced whey protein/κ-casein complexes as a means to investigate the acid gelation of milk—a review

“…Various authors have further reported that soluble whey protein/κ-casein complexes having a higher proportion of κ-casein were produced on heat-treating milk at slightly increased pH values from 6.5 up to 8.1 (Anema 2007;Donato and Dalgleish 2006;Menard et al 2005;Renan et al 2006). These milks also exhibited an increased pH of acid gelation, typically +0.2-0.4 pH unit (Anema et al 2004;. Among other causes and according to our interpretation, it may be that a higher content in κ-casein renders the soluble complexes more prone to interact with the casein micelles, where κ-casein initially belongs, thus accelerating gelation.…”

“…closer to pK SH/S−~9 ) has been shown to increase the proportion of intermolecular disulphide bonds in the complexes (Hoffmann and van Mil 1999). In skim milk, an increased proportion of covalent disulphide bonds in the heat-induced whey protein/κ-casein complexes produced at pH up to 7.3 (Donato and Dalgleish 2006) could partly account for the higher elastic modulus of the resulting acid gels (Anema et al 2004). On the contrary, blocking the free thiol groups of WPI with N-ethyl-maleimide (NEM) prior to heating yields complexes that fully dissociate in presence of SDS (Hoffmann and van Mil 1997).…”

Formation of heat-induced protein aggregates in milk as a means to recover the whey protein fraction in cheese manufacture, and potential of heat-treating milk at alkaline pH values in order to keep its rennet coagulation properties. A review

“…Other studies showed a strong relationship between the pH of milk and the proportion of β-lactoglobulin attached to the casein micelles upon heating. In skim milk heated at 90°C, 70-80% w/w of the β-lactoglobulin was bound to the micelles at pH 6.5, versus 30% w/w at pH 6.7, and 10% w/w at pH 7.1 [4][5][6]. In skim milk heated at 80°C, 60% w/w of the β-lactoglobulin was bound to the micelles at pH 6.55, versus 30% w/w at pH 6.9 [156].…”